The impact of chemical nematicides on entomopathogenic nematode survival and infectivity

Entomopathogenic nematodes (EPNs) belonging to the genera, Steinernema and Heterorhabditis, occur naturally in the soil along with plant-parasitic nematodes which are important root pests of many different crops. Here, we report the effects of four registered nematicidal compounds (fluopyram, fosthiazate, metam potassium, and fenamiphos) that are used for the control of PPNs on the survival, virulence, penetration efficiency, and reproduction of S. carpocapsae and H. bacteriophora. Despite previous studies warning of the impact of nematicidal compounds on IJ survival and infectivity, none have assessed their impact on EPN chemotaxis, penetration into and reproduction in insect host or conducted longer term soil assays. Survival of EPNs, based on observing IJ movement under a stereomicroscope after incubation in different concentrations of nematicides, showed that ≥ 80% of both nematode species were killed by fosthiazate, fenamiphos and metam potassium within 24 h. The recommended concentration of fluopyram killed 33% of H. bacteriophora, and 28% of S. carpocapsae after 48 h exposure. IJs exposed to the nematicides were less virulent against Galleria mellonella larvae in sand bioassay to non-treated IJs as significantly more control IJs of both EPN species penetrated their insect host (~47% of IJs added) than IJs exposed to fluopyram and fosthiazate; and the number of IJ progeny emerging from these controls was significantly higher than the number of IJ progeny emerging from IJs treated with the nematicidal compounds. In a highly novel discovery, the chemotaxis experiments using Pluronic F-127 gel indicated that H. bacteriophora were repelled from nematicide-treated environments. Moreover, IJs of both species added to treated natural loamy-clay soil at 5-day-intervals for 30 days post application of nematicides were generally unable to induce G. mellonella mortality, except for S. carpocapsae IJs added to fosthiazate-treated soil. Overall, our study indicates that there were detrimental effects of the nematicides on EPN IJs in treated soil. Therefore, overlap in timing for control of PPNs and the use of EPNs for biological insect control must be avoided. Future studies should assess the optimization of timing for nematicides use and survival and infectivity of IJs in the soil.